This invention relates to an improvement in systems for measuring the position and orientation of probes and other rigid bodies being tracked in 3-dimensional (3-D) space. The improvements engendered by this invention can be applied to 1-dimensional and 2-dimensional measurement systems, but the description herein describes the invention in the more general context of measurements in three dimensions.
Various methods and systems exist in the art to track the locations of points (markers) in a spatial volume defined by some 3-D coordinate system. By attaching multiple markers to bodies, even moving bodies, the orientation as well as the position of the bodies, individually or in relationship to each other, can be determined. For example, such bodies may be hand-held probes, moveable rigid objects, or semi-rigid portions of human anatomy.
Hereinafter, the position of a body means its 3-D location plus its 3-D orientation about that location. One common way of expressing this is as X, Y, and Z location coordinates and as yaw, pitch, and roll orientation angles. This is often referred to as six-dimensional information, or six degrees of freedom.)
A number of these methods and systems have been described in previous literature and have been used in practice. The description below will concentrate on light based-electronic tracking methods which use two or more light based sensors to measure the angular locations of markers on an object being tracked with respect to known positions of the sensors in the three dimensional volume. Examples of such prior art techniques are found in the following disclosures, which are herein incorporated by reference:
H. Fuchs, J. Duran, B. Johnson, and Zvi. M. Kedem; xe2x80x9cAcquisition and Modeling of Human Body Form Dataxe2x80x9d, Proc. SPIE, v. 166, 1978, p 94-102.
Jean-Claude Reymond, Jean-Luc Hidalgo; xe2x80x9cSystem for monitoring the movements of one or more point sources of luminous radiationxe2x80x9d, U.S. Pat. No. 4,209,254, Jun. 24, 1980.
Y. Yamashita, N. Suzuki, M. Oshima; xe2x80x9cThree-Dimensional Stereometric Measurement System Using Light based Scanners, Cylindrical Lenses, and Line Sensorsxe2x80x9d, Proc. SPIE, v. 361, 1983, p. 67-73.
F. Mesqui, F. Kaeser, and P. Fischer; xe2x80x9cReal-time, non-invasive recording and 3-d display of the functional movements of an arbitrary mandible pointxe2x80x9d, SPIE Biostereometrics 602, 1985, p 77-84.
Sharon S. Welch, Kevin J. Shelton, and James I. Clemmons; xe2x80x9cLight based position measurement for a large gap magnetic suspension systemxe2x80x9d, Proc. of the 37th International Instrumentation Symposium, San Diego, May 5-9, 1991 p. 163-182.
Faruhad Daghighian; xe2x80x9cLight based position sensing with duolateral photoeffect diodesxe2x80x9d, Sensors, November, 1994 p. 31-39.
Robert P. Burton and Ivan E. Sutherland; xe2x80x9cTwinkle Boxxe2x80x94A three-dimensional computer input devicexe2x80x9d, AFIPS Conference Proceedings 43, 1974, Chicago, Ill.
The markers in the above systems emit energy, and typically each marker is an active light source, such as an infrared or visible light emitting diode (LED). Other systems have been constructed to track highly reflective passive markers, and typically each such passive marker is a small patch or sphere coated with retro-reflective material, like that used on highway signs. By illuminating these markers with a light source near the sensors, a larger than normal amount of light is reflected back from the markers to the sensors, making the markers appear brighter than the background or other objects, thereby increasing their visibility and simplifying the process of finding them. Examples of commercial passive 3-D position measurement systems are the following:
The Vector vision system by BrainLAB GmbH (Heimstetten, Germany)
The Peak Motus system by Peak Performance Technologies, Inc. (Englewood, Colo.)
The Eagle Eye (trademark) system by Kinetic Sciences (Vancouver, British Columbia). However, the limiting problem in all such systems, whether using active or passive light based markers, is maintaining line-of-sight between the markers (reflectors or emitters) and the multiple sensors.
A number of non-light based 3-D measurement systems are known that do not present the line-of-sight limitations. For example, coordinate measurement machines (CMMs) and jointed mechanical arms do not require the marker to be within line of sight of the sensors, but they do require the tactile accessibility of a probe through rigid mechanical linkages, and this generally presents as much of a restriction on the accuracy and ease of operation as line-of-sight limitations. Further, these mechanical means are generally slower and more awkward to use than light based systems. For example, Carl Zeiss IMT Corp. (Minneapolis, Minn.), Romer Inc. (Carlsbad, Calif.), and FARO Inc. (Lake Mary, Fla.) manufacture such systems.
Other three-dimensional measurement systems that avoid the line-of-sight limitations include magnetic field based systems manufactured and sold by Polhemus Inc. (Colchester, Vt.) and Ascension Technology Corp. (Burlington, Vt.). The major drawback to such systems is that their accuracy is considerably degraded by the proximity of conductive objects, especially ferrous metals, and most especially large masses of ferromagnetic, materials, such as X-ray machines and other operating room apparatus. See also U.S. Pat. Nos. 3,983,474, 4,017,858, 5,453,686, and 5,640,170. Improvements have been attempted employing combinations of light based and other mensuration systems. The following reference describes one such combination of a light based and a magnetic tracking system for an image guided surgery application:
Wolfgang Birkfellner, Franz Watzinger, Felix Wanschitz, Rolf Ewers, and Helmar Bergmann; xe2x80x9cCalibration of Tracking Systems in a Surgical Environmentxe2x80x9d, IEEE Transactions on Medical Imaging 17, (to be published November 1998).
This reference describes calibrating a magnetic system for local anomalies by reference to a light based system before use of the magnetic system. It does not disclose continuous, dynamic registration of multiple (e.g. two) tracking systems during application, as will be discussed below. Furthermore, it does not describe light based tracking of the magnetic system""s field source generator.
One very desirable tracking system incorporates at least three built-in, orthogonal, miniature accelerometers and at least three built-in, orthogonal, miniature gyroscopes (or their equivalents) operatively associated with a probe or other tracked body. Such a system is desirable because it assumes that the accuracies of the accelerometers are very precise and that their operations are very stable over time. Unfortunately, to determine the absolute position and angular orientation of the probe or other objects in three dimensional space, their linear and angular acceleration must be integrated twice with respect to time. Furthermore, the slow rotation of Earth continuously affects all the angular measurements that are unaligned with the Earth""s poles. In other words, any tiny constant calibration error in the acceleration quickly accumulates into an unacceptably large error. Therefore, the sensors must be recalibrated very frequently, perhaps every minute or two, in order for their accuracy to be acceptable, particularly for medical tracking applications. Therefore, by itself, an inertia-based mensuration system is not very practical for submillimeter measurements and the minute accelerations experienced by a hand-held probe. However, a sufficiently accurate, inertia-based probe would be practical, if it could be recalibrated frequently (or better yet continuously) using some independent reference system.
Therefore, this invention presents an improvement in position measurement that combines the precision and robustness of light based tracking with another tracking system that does not have the xe2x80x9cline of sightxe2x80x9d limitations, such as: magnetic or inertial tracking, or ultra-sound, or any combination thereof. The result is a mensuration system that improves the accuracy or freedom of movement of the tracking system combination as compared to any of the individual tracking technologies alone.
The first objective of the present invention is to track the location of a probe or other object using a plurality of physical methodologies in such a way that results achieved by the combination are better than the results achieved using any one of individual constituent methodologies.
A second objective of this invention is to provide an automatic means to use a constituent methodology that has the best accuracy at any particular point in time and/or in space to continuously or frequently recalibrate the other constituent methodology (or methodologies) that have less accuracy at that point.
A third objective of this invention is to provide the operator of the system with a warning when the estimated inaccuracy position and orientation of the probe, or other tracked body or object, exceeds a prescribed limit.
Other and additional objects will become apparent from a consideration of the entirety of this specification, the attached drawing and the appended claims.
To meet these and other objectives, one aspect of the invention comprises a system for tracking the position and orientation of one or more bodies comprising multiple sensors that sense position and orientation and/or movement of the bodies using more than one form of physical phenomena, an associated control unit, and means (preferably automated) to perform geometric computations.
The following paragraphs describe the present invention in terms of two preferred embodiments that employ specific means and a specific method for determining the position and orientation of moveable, substanitially rigid bodies in 3-D space. Alternative means and methods are also mentioned in the text, but other unmentioned, comparable means and methods exist or will be developed that can implement the methods of the invention. All of such comparable means are intended to be embraced by this invention.